I've been playing with my 4S this morning and sharing texts with a fellow 4S owner. We've noticed something interesting:

The first time you SMS a 4S, the message is in a green bubble and it says 'sent as text message' below it. When they reply, it comes as an iMessage, and you can also reply via iMessage from that point. If you have a break in the conversation for some time (maybe 5 mins?) your next message drops back to being SMS.

It looks like the SMS must carry some kind of meta data which the receiving device responds to and initiates an iMessage session. Anyone know how they might be doing that?

How strong is your data signal? I noticed texting with my wife that one message in the middle of our conversation dropped back to SMS and the next one was back to iMessage. I bet she lost data connection momentarily so it auto switched and then came back when the connection was restored.

Definitely could be something like that - my friend and I kept texting each other throughout the day and later on it would initiate messaging via iMessage even if we hadn't exchanged messages for some time.